U.S. Pat. No. 5,022,225 discloses a method which especially relates to the transition from overrun operation to normal operation wherein the engine has a catalytic converter and at least one exhaust-gas probe mounted rearward of the catalytic converter.
The exhaust-gas probe in this patent is arranged rearward of the catalytic converter and is referred to as the rearward exhaust-gas probe in the following. The rearward exhaust-gas probe at least participates in the control of the fuel/air ratio. Dead times occur here which are caused on the one hand by simple gas-run times between mixture formation in the intake pipe and the exhaust-gas probe arranged in the exhaust-gas pipe but, on the other hand, also because of the oxygen storage of the catalytic converter. The catalytic converter still supplies oxygen for a certain time span to the exhaust gas when there is a change of the mixture composition from, for example, a lean mixture (oxygen rich) to a rich mixture (oxygen poor) so that the rearward exhaust-gas probe senses the mixture change after a dead time has passed. This dead time is especially long after an overrun phase of operation wherein the metering of fuel has been cut off. The control intervention, which is based on the signal of the rearward exhaust-gas probe, can lead to an unwanted intense enrichment of the mixture in this situation with a corresponding deterioration of the quality of the exhaust gas. The method of the above-mentioned patent tries to avoid this situation in that the control intervention of the rearward probe is suppressed for a predetermined time after an overrun phase of operation or control is continued with a value which was stored at the start of the overrun operation.
It has been shown that the quality of the exhaust gas after the overrun operation is not deteriorated exclusively by the described overreaction of the control. For example, an increased emission of nitrogen develops after an overrun phase of operation whereas the described excessive enrichment of the mixture leads to increased carbon dioxide and carbon monoxide emissions during operations which follow the overrun operation.